Stability constants absorbance measurements

The procedure used in this example begins with the preparation of a series of solutions, all containing the same concentration of metal ion and the same concentration of ligand, but having different low pH values. The absorbance of each solution is measured a plot of absorbance versus pH provides a curve that shows the extent of formation of the complex as a function of pH, similar to Figure 22-16. From this data, the stability constant can be calculated in the following manner. 

This method is apphcable for the determination of stability constants. The absorbance of two solutions with different metal to ligand ratios are measured at a wavelength where only the complex absorbs strongly, but the ligand and metal ion do not. The solution showing higher absorbance is... 

A common technique for measuring the values has been to employ species that produce anions with useful ultraviolet (UV) or visible (vis) absorbances and then determine the concentrations of these species spectropho tome trie ally. Alternatively, NMR measurements could be employed, but generally they require higher concentrations than the spectrophotometric methods. A hidden assumption in Eq. 5 is that the carbanion is fully dissociated in solution to give a free anion. Of course, most simple salts do fully dissociate in aqueous solution, but this is not necessarily true in the less polar solvents that are typical employed with carbanion salts. For example, dissociation is commonly observed for potassium salts of carbanions in DMSO because the solvent has an exceptionally large dielectric constant (s = 46.7) and solvates cations very well, whereas dissociation occurs to a small extent in common solvents such as DME and THE (dielectric constants of 7.2 and 7.6, respectively). In these situations, the counterion, M+, plays a role in the measurements because it is the relative stability of the ion pairs that determines the position of the equilibrium constant (Eq. 6). 

Hydroxamate- or catecholate-containing siderophores are strongly absorbing species with characteristic spectra (see Table 1) which can be utilized for spectrophotometric determination of the complex formation constant. Iron(III) hydroxamates absorb in the visible region, producing a broad absorption band in the 420-440 nm region. Iron(III) catecholates exhibit pH-dependent absorption maxima. Unfortunately, the overall Fe + ion complex formation constants cannot be determined directly at neutral pH, because the extremely high stability of siderophore complexes precludes direct measurements of the equilibrium of interest, which would yield the desired formation constant for a tris-bidentate siderophore complex, /3no (equation (2)). ... 

A frequently used approach to study the thermal stability of proteins is to incubate a protein solution at an elevated constant temperature and to observe the change of certain physical parameters (e.g., CD, IR absorbance, enzyme activity) over periods of minutes or hours. Such measurements deliver precious information for the practical application of the protein in question. On the other hand, it is impossible to extract thermodynamic or structural parameters firom such measurements, as they reflect the loss of native protein caused by a variety of processes. Irreversible thermal denaturation involves complex mechanisms and can lead to precipitation. The rates of such reactions depend on the concentration the rate constants depend on temperature and solution conditions. The order of such reactions can vary from 1 to FTIR has the advantage that it at least allows clear identification of -aggregation in the changes in the amide I band (1600-1700 cm ) of the infrared spectrum. The band component at around 1618 cm reliably reflects the progress of P-aggregation. ... 

The first step, R6, converts the HALS initially added to the clearcoat, parent HALS, into inhibition cycle, R7 and R8, products. These reactions compete with R2 and R3 lowering the stationary radical concentration, which in turn lowers the hydroperoxide concentration and the photooxidation rate. The rate constants and radical concentrations are such that only a small fraction (—5%) of the HALS stabilizer is in the form of nitroxide. Although nitroxides are thermally stable, they are not pho-tolytically stable. Nitroxides absorb light, and excited-state nitroxides can abstract hydrogen atoms to initiate free-radical formation. These reactions have been discussed in detail. "Reactions R9 and RIO are important both for the nitroxide decay measurement of free radical formation and in limiting the ultimate effectiveness ofHALS.i° i5... 

The efficiency of PEDOT PSS to stabilize individual SWCNTs in water, without the presence of low molar mass surfactants like SDS, has been shown using a UV-Vis spectroscopy method developed by Grossiord et al. This method was used to determine the optimal [PEDOT PSS) SWCNT ratio. The maximum achievable SWCNT exfoliation was achieved with a (PEDOT PSS) SWCNT ratio of 1 4. The final absorbance level observed in UV-Vis absorption spectra of dispersions after completion of the dispersion process was slightly higher for dispersions prepared with PEDOT PSS as compared to control SDS dispersions. This is most likely linked to a change of the dielectric constant value (s) due to the presence of a new medium in the vicinity of the nanotubes (shifts in absorption spectra are possible in a new chemical environment). Assuming 100% SWCNT exfoliation, the value for of the SDS-stabilized SWCNT dispersions, before 7i-plasmon subtraction, was determined to be 46.4 ml mg" cm at 500 nm, which is similar to reported values. It should be kept in mind that the UV-Vis absorbance spectrum of the PEDOT PSS itself is likely to be influenced by the presence of the SWCNTs. This makes quantitative analysis of these spectra impossible since the final absorbance is not simply a summation of the absorbance of the constituents measured independently (unlike exhibited for... 

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